1. Field of the Invention
The present invention relates to a tandem-type color image forming device, such as a laser printer, a digital copier or a facsimile device, in which color component images are written to respective photoconductors through light beam scanning and a color image is formed on an image support medium through superimposing of the color component images. More particularly, the present invention relates to a color image forming device which is provided with a correction-pattern image forming unit adapted for correction of the image formation operating states for each color component.
2. Description of the Related Art
In recent years, in image forming devices, such as a printer, a digital copier and a facsimile device, which perform image formation by using the electrophotographic process, the light scanning method which performs the optical image writing to the photoconductor by the scanning of a light beam (e.g., laser beam) is commonly used. In this light scanning method, the photoconductor is periodically scanned in the main scanning direction by the scanning unit, such as a polygon mirror, through the scanning of a laser beam the light emission control of which is performed in accordance with a video signal (line image signal). And the scanned surface of the photoconductor is moved in the sub-scanning direction (which is perpendicular to the main scanning direction). A two-dimensional image is formed on the photoconductor by performing the exposure scanning.
Subsequently, the electrostatic latent image formed on the photoconductor by the exposure scanning is subjected to each of respective processes of the development using toner, the image transfer to a recording medium or copy sheet (which may include an intermediate transfer medium), and the fixing of the image to the recording medium. After these processes, the image formation processing is completed.
When a color image is formed using the light beam scanning method, the scanning of a light beam to the photoconductor is performed for each of respective color components, and a color composite image is produced through the superimposing the color-component images. Regarding this processing, there are known the two major methods. One is the single-photoconductor method in which the color superimposing is performed in the optical writing or image transfer process using the single photoconductor that is common to each color component. The other is the tandem type method in which the color superimposing is performed in the image transfer process using a plurality of photoconductors corresponding to the respective color components.
In the tandem type method, the exposure scanning is performed to the photoconductor of each color component respectively, and then the color superimposing is performed. And it is necessary to manage the image formation process so as to prevent occurrence of deviations between the respective color component images. For this reason, it is necessary to output an appropriate color image by measuring or detecting the image formation state of each of the color component images and adjusting the operating conditions in accordance with a detected change of the image formation state.
Japanese Published Application No. 07-019085, Japanese Patent No. 3644923, and Japanese Laid-Open Patent Application No. 2004-101567 disclose examples of the operating-state measurement method according to the related art which is used for the tandem type method.
The measurement method of Japanese Published Application No. 07-019085 is to measure a color deviation in the copy sheet transport direction by forming a pattern image of each color on the transport (transfer) belt, on the conditions that it is formed in the transport direction at predetermined intervals during operation without any error, and by detecting a change in the pattern image. That is, the pattern image of each color actually formed at the time of measurement reflects variations in the image formation operating states for each color and includes a positional deviation of the interval between the pattern images. This deviation is detected by a sensor, and the image write timing is adjusted in accordance with the detected signal from the sensor.
The measurement method of Japanese Patent No. 3644923 is based on the above-mentioned method of Japanese Published Application No. 07-019085 wherein the pattern image of each color is formed on the transport (transfer) belt. In this method, in addition to the positional deviation between the pattern images of the respective colors, other deviations, due to errors of a sub-scanning registration (or the above-mentioned deviation in the copy sheet transport direction), an inclination (skew), a main-scanning registration and a scanning magnification, are also included. For this reason, a sequence of positioning toner marks for detecting a deviation is formed at three detection positions on the transport belt arrayed in the main scanning direction.
Moreover, in the method of Japanese Patent No. 3644923, the optical density detection toner mark (patch) for optical density detection of each color is also formed, and the detection unit for detecting the positional deviation is shared for detection of this optical density detection toner mark.
In the measurement method of Japanese Laid-Open Patent Application No. 2004-101567, the processing of the detection data which optimizes positioning is performed on the basis of detection of the positioning toner marks for detection of positional deviation between the images of each color as in the method of Japanese Patent No. 3644923.
Based on the data which represents the measurement result of the operating state acquired by the measurement method as disclosed in Japanese Patent No. 3644923 or Japanese Laid-Open Patent Application No. 2004-101567, the correction is carried out and the operating conditions are adjusted so that a high-quality image without color deviation can be formed.
The above adjustment is carried out for the exposure scanning unit by adjusting the timing of image writing, the drive of the photoconductor or the amount of light exposure. Or the above adjustment is carried out for the toner development unit by adjusting the development bias or the charging bias. Since the state of the system changes temporally, the above adjustment must be performed at appropriate timing.
In the measurement method which detects the image formation operating state by measuring the toner marks, in order to derive various kinds of correction (adjustment) values of the respective colors or those needed between the respective colors from the detection result of the toner marks on the transport (transfer) belt by means of the sensor, the toner marks on the transport (transfer) belt are formed in accordance with the predetermined conditions for this purpose.
For example, FIG. 11 shows the arrangement of toner marks for detection of positional deviation between the respective colors according to the art related to the invention. As shown in FIG. 11, a mark sequence 17′ which includes four lateral lines and four slanting lines of the respective colors arranged at predetermined intervals is set up as one group, and this mark sequence 17′ is formed at each of detection positions of the sensors 14, 15 and 16 which are disposed on the transport belt at three different locations in the main scanning direction.
The mark sequence 17′ shown in FIG. 11 is similar to the deviation detection toner marks as disclosed in Japanese Patent No. 3644923 or Japanese Laid-Open Patent Application No. 2004-101567. The letters M, C, Y and K indicated in the mark sequence in FIG. 11 denote the respective color components (M: magenta, C: cyan, Y: yellow, K: black).
The mark sequence 17′ (or deviation detection marks) is formed on the transport belt during a special operation mode (which is called correction mode) which is performed to correct the image formation operating states, and this correction mode is different from the normal printing mode (which is also called normal printing) which is performed to form an image on a copy sheet.
In the tandem type color image forming device according to the related art, the toner marks are formed on the transport belt in the sequence: M-C-Y-K, as shown in FIG. 11, along the belt transport direction.
In the tandem type color image forming device according to the related art, the photoconductor drums of the respective color components are arranged in the sequence of M-C-Y-K in the direction from the upstream to the downstream of the transport belt, and the marks of the respective colors are assigned to the image formation areas of the respective colors arranged in a sequence that is the same as the sequence of the photoconductor drums in the above-mentioned arrangement.
FIG. 12 shows the arrangement of the image formation areas on the transport belt to which the toner marks of the respective colors are assigned according to art related to the invention. As shown in FIG. 12, the uppermost position in the mark sequence upstream of the belt transport direction is set to M. The area “a” (where “a” denotes the length of the mark in the belt transport direction) is assigned for each of the respective colors along the sequence of M-C-Y-K, respectively, and the mark of each color is formed therein. And the mark sequence in the belt transport direction is constituted in this manner.
Similarly, with respect to the optical-density detection mark (patch), the area “a” is assigned for each of the respective colors.
FIG. 13 is a timing chart for explaining the image formation area signals which cause the toner marks of the respective colors to be formed in the assigned image formation areas.
With respect to each of the image formation area signals of FIG. 13, the Low period is the write-enable period in which image formation is possible, and the shaded rectangular signal portion is the period (assigned for image formation) in which the toner mark of the color concerned is formed on the transport belt.
In FIG. 13, it is assumed that sub-scanning (belt transport) is performed at a constant speed and the period in the timing chart is considered a linear distance (length). And the image formation area length (or the write-enable period) is represented by “4a” (mm), and one fourth “a” (mm) of the image formation area length is assigned for each of the respective colors M, C, Y and K, as the shaded rectangular signal portion.
The pitch between two adjacent ones of the photoconductors of the respective colors is set to “b” (mm), and the timing of each image formation area signal is adjusted so that the toner marks of the respective colors are respectively formed in the assigned image formation areas on the transport belt.
As shown in FIG. 13, according to the related art, upon start of the mark formation, the photoconductor of M arranged in the uppermost position upstream of the belt transport direction is set in the write-enable period in which image formation is possible, and the mark of M is formed in the head-end image formation area on the transport belt.
Subsequently, the period of the photoconductor pitch “b” is delayed from the start, the photoconductor of C arranged in the second uppermost position upstream of the belt transport direction is set in the write-enable period in which image formation is possible, and the mark of C is formed in the second image formation area on the transport belt. Similarly, the mark of Y is formed in the third image formation area on the transport belt.
Subsequently, the period “3b” is delayed from the start, and the final mark of K is formed in the last image formation area on the transport belt.
Therefore, according to the related art, the total period “4a+3b” is needed from the start of formation of the first mark of M to the end of formation of the last mark of K.
The correction mode is automatically performed if a print request is received from the operation panel by the user and a change of the image formation operating state of the image forming device which degrades the image quality, such as a color deviation, takes place. For example, such a change may take place when printing documents more than a predetermined number of sheets is performed, or the image forming device starts operation from the idle state, such as power supply ON, or a temperature change arises which causes the operating state of the device, such as the exposure scanning unit, to change.
The above problem will become the hindrance of quick document printing, and the user who desires to obtain printed documents as early as possible will feel dissatisfaction, and the productivity will be reduced.
Therefore, in order to meet the demand for a quick image formation processing and suppress the fall of productivity, it is desirable to shorten the time needed for forming the toner marks.